Phenol-formaldehyde (PF) resins and especially PF resins extended with urea (PFU) find widespread use as adhesives and bonding agents for making a wide variety of products, especially non-woven fiber products such as fiberglass insulation.
Indeed, PF and PUF resins have been the mainstays of fiberglass insulation binder technology over the past several years. Such resins are relatively inexpensive and provide cured fiberglass insulation products with excellent physical properties.
One of the complications presented by using conventional PF and PFU resins in formulating binder compositions from such resins is the presence of bis (4-hydroxy-3,5-dimethylolphenol)methane (also known as tetradimer) in the resin compositions. Tetradimer is generally present in conventional PF resins at a concentration of about 10 to 18% by weight (adjusted for a typical resin concentration of 50% solids).
The problems presented by the presence of tetradimer in PF and PFU resins are well documented. When these resins are diluted with water during the preparation of a binder composition (particularly when the formaldehyde content is reduced below about 2-3 weight percent), the tetradimer is prone to precipitation in tanks and piping and posses a plugging problem in binder application equipment, such as sprayers. Consequently, as efforts to reduce formaldehyde concentration and accordingly formaldehyde emissions in such binders increase, problems created by tetradimer precipitation are exacerbated. Unfortunately, the tetradimer crystal phase is very difficult to re-dissolve and often must be removed and discarded, increasing production expenses and decreasing binder efficiency. Thus, care must be taken to accommodate the presence of tetradimer in these resins to avoid production down time for cleaning the un-welcomed precipitate.
The prior art has sought ways of minimizing tetradimer production during the synthesis of PF resins. Higginbottom U.S. Pat. No. 4,028,367, for example, describes an aqueous resole resin composition that is purportedly stable with respect to the unwanted crystallization of tetradimer and also is purportedly low in free phenol (P) and free formaldehyde (F).
According to the Higginbottom patent, the aqueous resole composition is prepared using a complicated two-step process. First, a molar excess of phenol (P) is reacted with formaldehyde (F) (1 mole phenol with 0.05 to 0.3 mole formaldehyde) under an acidic condition sufficient to form a novolac resin. Thereafter, the resin is neutralized and then made alkaline as additional formaldehyde is added (broadly described as 1.75 to 3.5 moles per mole of original phenol), and reacted under basic conditions to yield the resole resin. The level of free formaldehyde in the resole resin is then reduced further by adding a formaldehyde scavenger near the end of the resole reaction in an amount of 0.5 to 1.5 mole equivalents per mole of free formaldehyde. Urea is one of several scavenger options disclosed.
The Higginbottom patent alleges that that the sizable population of 2,2′- and 2,4′-dihydroxydiphenylmethanes that is produced, along with a minor amount of the tetradimer, helps to suppress crystallization of the tetradimer and allows the level of free (unreacted) formaldehyde to be reduced almost completely in the resole, often through the use of a formaldehyde scavenger.
The resole resin obtained by the process of the Higginbottom patent is said to have a water tolerance in the range of 100 to 800 percent, i.e., haze occurs when an amount of water from 1 to up to about 8 times the amount (mass) of resole is added to the resin. This latter property constitutes an additional impediment to the widespread use of the Higginbottom resole resin for making fiberglass insulation as higher levels of dilution are generally preferred when making a fiber mat binder. Indeed, it is preferred that the resin exhibit an infinite water dilutability, which is considered to be a dilution ratio of at least 50 parts water to one part resin.
U.S. Pat. No. 4,757,108 purports to produce a PFU resin of improved storage stability against tetradimer precipitation by using a carefully controlled acidification reaction to form urea-formaldehyde (UF) polymers of limited molecular weight that are fully water soluble. With the pH initially on the alkaline side, urea is added to the PF resole resin for reaction with free formaldehyde present. The pH is then adjusted to be acidic and is maintained in an acidic pH range for a limited time at a slightly elevated temperature. At the end of the limited time, the resin solution is neutralized, allowed to cool, and is then ready for storage. The UF polymers so-formed allegedly inhibit the crystallization of tetradimer.
U.S. Pat. No. 5,623,032 also purports to produce a PF resin of improved storage stability against tetradimer precipitation by using one member of a particular class of tertiary amine alcohols as a catalyst.
U.S. Pat. No. 6,881,814 alleges that the addition of a small amount of sodium tetraborate early in the preparation of a PF resin reduces the amount of tetradimer formed and improves the stability of both the base resin and its pre-react (PFU).
U.S. Pat. No. 6,906,130 also reportedly produces a resin low in tetradimer concentration. Again, a two stage process is described in which a novolac resin first is prepared at a low F:P mole ratio (0.01:1 to 0.3:1) preferably using a strong acid catalyst. Following neutralization, a conventional alkaline catalyst then is added and additional formaldehyde is added thereafter as quickly as possible to the reaction solution while maintaining the temperature at or below 55° C. A key feature of the invention is that the temperature during the resole reaction is limited to a temperature below about 60° C., in order to make a low molecular weight resin and obtain maximum methylolation of the phenolic core, rather than causing linking condensation reactions that build molecular weight. The resulting resin has a large free formaldehyde concentration, which then is reduced by use of a formaldehyde scavenger, preferably urea, before using the resin for formulating a binder composition.
Notwithstanding these approaches, there remains a need for newer, less complicated methods for addressing the problems presented by tetradimer formation when preparing a phenol-formaldehyde resin suitable for making a binder composition for non-woven fiber products, such as fiberglass insulation.